A large amount emission of carbon dioxide caused by massive fossil fuel-fired power generation has been an environmental issue for the globe warming. Before an efficient solution of the requirement for fossil fuel is provided, carbon dioxide capture and storage (CCS) technology is an important method known up to date for reducing carbon dioxide emission. Such method is also certified by the Intergovernmental Panel on Climate Change (IPCC) as an efficient mechanism for reducing the greenhouse gases.
According to the methods for converting fuel to heat and electricity, current carbon dioxide capture techniques can be grouped into three categories: i.e., post-combustion capture, pre-combustion capture, and oxy-combustion capture (oxyfuel). Chemical-looping combustion process is classified as a special case of oxy-combustion, and it is particularly promising to execute the process by using interconnected fluidized beds. This process exhibits the features that include low emissions and high efficiency, making it recognized as full of potential for development.
The principle of chemical looping is to feed fuel into a fuel reactor for proceeding reduction reaction with the added oxygen carrier MexOy at 900-950° C. The fuel is thus oxidized to CO2 and H2O, while MexOy is reduced to MexOy−1. The reduced MexOy−1 is then sent into an air reactor for proceeding oxidation reaction with oxygen at 500-700° C. and converted back to MexOy; hence, a looping process is formed. Accordingly, as the oxygen carrier is circulating in the chemical looping process, carbon dioxide and steam vapor are produced in the fuel reactor. By separating the vapor through a cooling unit, it can acquire carbon dioxide with purity higher than 95%. Besides, heat is also acquired in the air reactor. The produced carbon dioxide can be reused or sequestrated directly. This method has the advantages of low cost and high efficiency.
The interconnected fluidized bed integrates a plurality of circulating fluidized bed and several solid transport pipes, in which the various reaction processes are accomplished by transporting solids among multiple fluidized beds at different fluidized speeds; for the sake of implementation, the beds are formed by combining at least two single beds. In the process of operation, the fluidized solids descend in a dense bed, pass through orifices at the bottom, and enter a lean bed. Then said solids ascend in the lean bed, surmount a weir at the top, enter another dense bed, and the cycle repeats. The interconnected fluidized bed has the advantages similar to a single circulating fluidized bed, but without complicated mechanical structures such as the solid transport pipes; thus, the solid circulation rate is high and the solid loss is less. Consequently, the interconnected fluidized bed reduces the costs in installation and operation, and its operation efficiency is higher than that of various traditional fluidized beds.
Although the interconnected fluidized bed saves massive transport pipes in the structure, there is currently lack of efficient technique for dynamically controlling the circulation rate of inner solid (fluid-like particles). Because the reaction rate in respective bed regions is only relied on and limited by the predetermined system standard, after the solids enter the interconnected fluidized bed, a user can at most alter the total feed amount of solids and the gas velocity. This limitation does not meet the requirements of industry, especially when linear adjustment on the circulation rate of solids cannot be performed effectively for finding the optimum circulation rate. Furthermore, with the conditions limited by altering the total amount of solids and the gas velocity, the functions of the interconnected fluidized bed cannot be utilized completely.